states are currently prohibited from collecting sales tax from online sellers who are not physically located in that state. >>> we're getting new information coming into to cnn now on the death of that m.i.t. police officer who was shot and killed by the tsarnaev brothers. what are you learning? >> we want to give you some perspective on what happened. the officer shawn collier, he did not radio into dispatch to request a diskripgs of the suspects. he did not have time to activate the alert on his radio. we are told he was gunned down between four and five times. it was what amounted to an execution. he was shot in the head and the chest. he was sitting alone in his squad car. it took boston police almost 13 minutes to find the vehicle and realize he was the one who had been injured. they were responding to 911 calls from other people in that vicinity. this officer never had time to alert his fellow colleagues where exactly he was. it is unclear where the two brothers ambushed this officer. did they panic? it is not clear. according to a source, they headed away, in the direction of